발간논문

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Chlorination behaviours of tungsten oxide (WO₃) were investigated by means of a thermogravimetric analysis. The rate of chlorination was increased with increasing the mixed ratio of carbon as a reducing agent. the flow. rate of chlorine gas, reaction temperature, and the amount of calcium fluoride as an additive. It was found that experimental results fit well Valensi equation, 1-2/3R-(1-R)^(2/3) = Kt and the rate of chlorination is controlled by a diffusion process, the activation energy of which is 26.0 KJ/mole. Discussion of the chlorination mechanism is also given in the text.

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The characteristics of SCC crack propagation in duplex stainless steel weldment made by SMAW GTAW and GMAW processes were investigated in 42% MgCl₂,142℃ boiling solution. From these experiments, it could be concluded that the strycture anisotrpy of y phase as well as the phase ratio played an important role in SCC resistance. GTA and GMA weld metal showed higher SCC resistance than base meta1 because of randomly distributed γ phase. The crack in weld metal had same opportunity of receiving keying effect as that in base metal, but it had less possibility of intersecting γ phase. The SCC resistance of the SMA weld metal and the HAZ was lower than that of the base metal because their phase ratio deviated from the proper phase ratio.

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Silica, magnesia-chrome and zircon bricks used in the R- H ladles of POSCO were made into crucibles, in which electrolytic iron and aluminium were maintained at 1600℃ for one hour under nitrogen atmosphere. The crucibles teated were cut into thin sections for optical microscopic and electron optic investigations to find the failure mechanisms of the different refractories in the prescence of the aluminium. Silica brick failed by the reduction of SiO₂ by Al added and the damage was aggravated by penetration of Al₂O₃ into brick. Attack of magnesia-chrome brick was characterized by spalling due to an anisotropy developed along the border outlined by penetration of the aluminium silicates into the crucible. Zircon brick remained almost intact due to ZrO₂ resulting from the decomposition of zircon and mullite formed as a reaction product of Al and refractory materials.

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The effect of δ-ferrite in the AlSI 304 austenitic stainless steel weldment on the stress corrosion crack propagation in boiling MgCl₂solution was investigated. The network structure of δ-ferrite retarded the crack propagation rate due to keying effect, and the crack propagated along the boundaries of δand δ-ferrite phases. However, the crack propagation rate of the weldment was even faster than that of the matrix when dissolved nitrogen content was so high that there was no δ-ferrite in the weldment. Comparing with the preciously published paper, we found thats the predominant factor determining the stress corrosion clacking life time was not the time for crack initiation, but the crack propagation rate. Application of the linear fracture mechanics to stress corrosion cracking was a1so discussed briefly.

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The present study concerns the effects of cerium, didymium contents, sintering, heat-treating conditions and microstructure on the magnetic properties of Ce-Didymium-Fe-B permanent magnets. Best magnetic properties were achieved for Didymium (30%Nd-20%Pr)-Fe-1%B and For 4%Ce-Didymium (80% Nd-l6% Pr-4% Ce)-Fe-l%B by using 35wt% of Didy mium and of 4%Ce-didymium, respectively and by sintering for 1 hour at 1100℃ and heat-treating for 1 hour at 600℃. The magnetic properties of Didymium-Fe-B magnets were better than those of 4%Ce-Didymium-Fe-B and 8%Ce-Didymium-Fe-B magnets but were inferior to those of Nd-Fe-B magnets when all magnet samples were fabricated under optimum processing conditions. The magnetic properties were discussed in relation to the microstructure consisting of R₂Fe_(14)B matrix, Nd-rich and Ce-rich phases along gram boundaries.

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The effect of the retrogression and leasing (RRA) treatment on the high strangth Al 7075 alloy was studied by measuring the hardness behavior and the propagation rate of the stress corrosion (SC) crack Transmission electron microscopy was used to investigate the variation of the microstructure of both the matrix and the grain boundary. The initial rapid decrease in the strength during the retrogression treatment is believed to be due to the partial or complete dissolution of small particles of η` transition phase. The RRA treatment results in a significant decrease of the SC crack propagation rate without deteriorating the original T6 strength of the alloy. This is believed to be due to a significant coarsening of grain boundary precipitates during the retrogression treatment. The RRA structure retains a high density of overall particles in spite of their significant coarsening.

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The microstructure and the fracture toughness of the weld-heat-affected zone of a ductile cast iron haute been investigated with a thermal cycle simulator to simulate the actual welding process. The matrix structure of the heat-affected zone transformed to martensite, mixture of martensite and pearlite, pearlite, and pearlite including small amount of ferrite, in turn, wish increasing cooling time from 800℃ to 500℃. The use of a proper preheat temperature prevented the formation of martensite. The welding condition for this involved a preheat temperature of 200℃ and a heat input of 30 KJ/㎝. Also, a post-weld-heat treatment of 675℃ for 2 hours was effective in reducing the hardness of ductile cast iron weldments by eliminating mariensite. The secondary graphite formed due to martensite decomposition at that title. An instrumented Charpy impact test has been carried out on a welded and post-weld-heat-treated ductile cast irons, respectively. Results showed that the dynamic fracture toughness increased slightly with higher preheat temperature and heat input. The post-weld-heat treatment improved the dynamic fracture toughness of the weldments to three times that of the as-welded condition.

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The mechanical properties of virgrin martensite, ausformed martensite, marformed martensite, tempered ausformed-martensite, tempered marformed-martensite and reversed austenite produced by phase transformation and thermomechanical treatment of Fe-30%Ni-0.2%C alloy have been investigated. As and A, point are increased with the amount of deformation and tempering temperature, the increase of A_s point in marformed martensite being especially great, The tempering resistance of marformed martensite is superior to that of ausformed martensite due to the increase in A_s point. The strength of reversed austenite transformed from ausformed martensite and marformed martensite is decreased at the high degree of deformation due to disappearance of considerable amount of strengthening effect introduced by thermomechanical treatment during the reverse transformation. The elongation in deformed austenite decreased with increasing deformation; those of ausformed martensite and marformed martensite, however, maintained constant values, independent of the strength. Consequently, marforming treatment is superior to ausforming for strengthening Fe-30%Ni-0.2%C alloy.

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A study has been made of residual stresses in the surface layer of heavily cold drawn 5056 aluminum alloy wire and 6253 aluminum alloy clad (clad cross section ratio: 10-30%) 5056 aluminum alloy wires. For this purpose, an X-ray stress analysis was applied. The residual stresses were evaluated under an assumption of isotropy. The 5056 aluminum alloy wire had tensile residual stresses. The same alloy wires, however, exhibited compressive residual stresses when they were clad in a softer 6253 alloy.

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The present study was fonducted to estimate the fatigue crack growth (FCG) rate and the fracture behavior of a low carbon 1.4% Mn steel (HT50) and its CO₂ gas welded metal. The investigation was made on two different directions of each material: the rolling direction and the transverse direction of the base metal, and the parallel direction and the perpendicular direction to the welding line. The fatigue crack growth rate of the base metal was higher than that of the welded metal and the heat-affected-zone (H.A.Z). Secondary cracks were observed on the fatigue fracture surface of all specimens and these were attributed to the reduction of the fatigue crack growth rate. The experimental values of the fatigue crack growth rate showed a good relationship with the strition spacing calculated from an empirical equation in the low △k region.

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